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Calcium Deficiency and Ammonia Toxicity as Separate Causal Factors of (NH₄)₂HPO₄-Injury to Seedlings¹

ABSTRACT

Short term experiments were conducted in a growth chamber with seedlings of sudangrass (Sorghum sudanensis) and cotton (Gossypium hirsutum L.) on a Lucedale sandy loam surface soil treated with different rates of MgO, CaO, (NH₄)₂HPO₄, K₂HPO₄, and Ca(H₂PO₄)₂. Sudangrass seed was planted in treated soils and the seedlings were allowed to grow for 14 days, during which time they were observed for symptoms of Ca deficiency or NH₃ toxicity. Cotton seed was planted in a layer of untreated soil that was underlain by treated soil in glass-front boxes for direct observation of root growth. The depth to which each primary root of cotton extended into the treated soil was measured periodically for 3 days after the root entered the treated soil. Results were related to ionic composition of soil solutions in situ. Calcium deficiency symptoms and reduced growth occurred on all soils with a_Ca/a_{cation i} ratios in soil solutions of 0.15 or less. Ammonia toxicity symptoms on foliage were caused by only one treatment although toxic concentrations of NH₃(aq) were present in three other soil solutions. These solutions were also deficient in Ca. When conditions conducive to Ca deficiency and NH₃ toxicity existed in the same soil solution, Ca deficiency symptoms dominated except at the highest NH₃(aq) concentration.

¹ Contribution from Department of Agronomy and Soils, Auburn University Agr. Exp. Sta., Auburn, Ala. Part of a dissertation submitted by the senior author in partial fulfillment of requirements for the Ph.D. degree.

² Assistant Professor of Soils and Professor of Soils, respectively.

Received for publication August 4, 1969. Accepted for publication October 24, 1969.